My script for T280731 has identified some messages from the wikitech-l pipermail archives that are not in hyperkitty on lists-next:
ERROR: https://lists.wikimedia.org/pipermail/wikitech-l/2002-March/000176.html not in hyperkitty: 20020308180628.GA3143@tavaiah (IBSQQ4WZHHOC4IU4VE2ONEPVKW23STPP) ERROR: https://lists.wikimedia.org/pipermail/wikitech-l/2002-November/005560.html not in hyperkitty: 001701c257f3$b3d86d10$0200a8c0@faloona.net (MATJ7SX3VRGIZW5HCNMCSJOQZNZ5OUOU) ERROR: https://lists.wikimedia.org/pipermail/wikitech-l/2003-June/004576.html not in hyperkitty: 20030628115222.GA9886@math-cl-n01.ucr.edu (OBUGNS2DNVMUQ63JMODCA7KNSHCRNL4J) ERROR: https://lists.wikimedia.org/pipermail/wikitech-l/2003-June/004402.html not in hyperkitty: 20030612161146.25323.qmail@web41713.mail.yahoo.com (6K2BUWPJBLSWCZOM5GRKYH24Y45ZKEIJ) ERROR: https://lists.wikimedia.org/pipermail/wikitech-l/2003-June/004401.html not in hyperkitty: 20030612160956.70036.qmail@web41709.mail.yahoo.com (LOXKXKXRUMGCBDNBQYNRJ72AUEH4VIUE) ERROR: https://lists.wikimedia.org/pipermail/wikitech-l/2003-November/006934.html not in hyperkitty: 1069095271.5585.0.camel@ william (FX267SRJAOZFWB7HN2FTKBAGYNQ4ROZC) ERROR: https://lists.wikimedia.org/pipermail/wikitech-l/2004-March/008972.html not in hyperkitty: 20040309154913.13088.qmail@web60501.mail.yahoo.com (UBP7L2PLV7654OSFLBYO6IWT4KC2QROD) ERROR: https://lists.wikimedia.org/pipermail/wikitech-l/2004-March/008960.html not in hyperkitty: 20040309110007.43418.qmail@web60503.mail.yahoo.com (XQKALEVBVTXAGZ6WORREOAO5DLNMWGGD) ERROR: https://lists.wikimedia.org/pipermail/wikitech-l/2004-July/011791.html not in hyperkitty: 20040730084056.56626.qmail@web41809.mail.yahoo.com (CKTTZZ2QI27MIITC5KYCUVANULG24KWY) ERROR: https://lists.wikimedia.org/pipermail/wikitech-l/2004-July/011354.html not in hyperkitty: 20040719095757.24522.qmail@web41803.mail.yahoo.com (6CWHTI4GAV53LRFK7OVRHWEERNIL4EL6) ERROR: https://lists.wikimedia.org/pipermail/wikitech-l/2004-July/011239.html not in hyperkitty: 20040715143046.91117.qmail@web41813.mail.yahoo.com (URVLRY7XGKRSPJ2ZZYBND6PK2DCQ7DA2) ERROR: https://lists.wikimedia.org/pipermail/wikitech-l/2004-July/011236.html not in hyperkitty: 20040715124740.29719.qmail@web41801.mail.yahoo.com (GRQJDPTDXMDV3PJSOI2WK5KYDGUV7L3Q) ERROR: https://lists.wikimedia.org/pipermail/wikitech-l/2004-August/011878.html not in hyperkitty: 20040803135927.9779.qmail@web41801.mail.yahoo.com (M4ADAN4MEGFCTJHZQHAMJSHPT63YBFAB) ERROR: https://lists.wikimedia.org/pipermail/wikitech-l/2005-January/014563.html not in hyperkitty: 200501042046.00692.yann@forget-me.net (GWGH5KWVYDTOP4WYSXCF7FPRW3PERU4J)
(it's still running, more might come)
The emails should be in hyperkitty as https://lists-next.wikimedia.org/hyperkitty/list/wikitech-l@lists-next.wikimedia.org/message/MESSAGE-ID-HASH/.